The present invention relates to body implantable devices and more particularly to prostheses incorporating the characteristics of stents and grafts and intended for long term intraluminal fixation.
A variety of patient treatment and diagnostic procedures involve devices intraluminally implanted into the body of a patient. Among these devices are stents as disclosed in U.S. Pat. No. 4,655,771 (Wallsten). The devices in Wallsten are tubular, braided structures formed of helically wound thread elements. The stents are deployed using a delivery catheter such as discussed in U.S. Pat. No. 5,026,377 (Burton et al.). With the stent positioned at the intended treatment site, an outer tube of the delivery catheter is withdrawn allowing the prosthesis to radially expand into a substantially conforming surface contact with a blood vessel wall or other tissue.
Thread elements or strands formed of metal are generally favored for applications requiring flexibility and effective resistance to radial compression after implantation. Metal strands can be thermally formed by a moderately high temperature age-hardening process while wound about a mandrel in the desired helical configuration. The strands, due to their high modulus of elasticity, cooperate to provide the needed strength. Strand flexibility also permits a radial compression and axial elongation of the stent that facilitates intraluminal delivery of the stent to the intended treatment site. Because the self-expanding device generally remains at least slightly radially compressed after fixation, its elastic restoring force can provide acute fixation.
The favorable combination of strength and flexibility is due largely to the properties of the strands after they have been age-hardened, or otherwise thermally treated in the case of polymeric strands. The braiding angle of the helical strands and the axial spacing between adjacent strands also contribute to strength and flexibility. Age hardening processes are described in U.S. Pat. Nos. 5,628,787 (Mayer) and 5,645,559 (Hachtman et al.).
A well known alterative stent construction features plastically deformable metallic strands in lieu of resilient strands. Plastically deformable strands can be arranged in the same helical configuration. A plastically deformable stent requires no gripping members or other feature on the catheter to maintain the stent in a reduced-radius state during delivery. However, radial expansion of the stent at the treatment site requires a dilatation balloon or other expansion means.
Regardless of whether stents are self-expanding or plastically deformable, they characteristically have an open mesh construction, or otherwise are formed with multiple openings to facilitate the necessary radial enlargements and reductions and to allow tissue ingrowth of the metallic structure. Also, such stents characteristically longitudinally expand as they radially contract, and conversely radially expand as they longitudinally contract.
Devices featuring more tightly woven strands are known. For example, U.S. Pat. No. 4,681,110 (Wiktor), discloses a flexible tubular liner insertable into the aorta to treat an aneurysm. The liner is a tight weave of flexible plastic strands, designed to elastically expand against the aneurysm to direct blood flow past the aneurysm. The tight weave is intended to minimize leakage, so that the liner can effectively shunt blood through to eliminate the aneurysmal sack from the blood path.
The Wiktor structure and others like it notwithstanding, those of skill in the art continue to encounter difficulty in providing a device that simultaneously accommodates the competing needs of low permeability, strength and flexibility for radial compression and expansion. One known approach to this problem is a combination stent graft, in which a compliant but substantially fixed-radius and tightly woven graft is sutured or otherwise coupled to a radially expandable stent. Upon release, the stent is intended to radially expand to the graft diameter. This requires a careful matching of the graft diameter with the lumen diameter at the treatment site. Otherwise, either an oversized graft is compressed between the stent and body tissue with undesirable folds or gathering of the graft material, or an undersized graft prevents the stent from radially expanding an amount sufficient to anchor the device.
Another difficulty arises from the fact that the stent layer and graft layer, even when both undergo combined radial contraction and axial elongation, behave according to different relationships governing the amount of radial reduction for a given axial increase. When the latticework elongates a greater amount for a given radial reduction, elongation of the composite structure tends to tear the bond joining the graft material to the stent. Conversely, if the graft layer undergoes the greater axial expansion, an unwanted increase in bending stiffness causes localized reductions in diameter when the stent graft is bent around tight radii. Consequently negotiation through tortuous vascular passageways becomes more difficult, and in some instances impossible.
The commercially available yarns used in textile vascular grafts are twisted for improved handling during weaving or knitting operations. The amount of twisting will depend upon certain factors including the process of yarn manufacture (e.g., continuous filament yarn or staple yarn) and desired denier. For continuous filament yarn processes, surface twisting angles will generally be between about 15-45 degrees. The multiple filaments typically form a substantially circular yarn cross-section. This limits the effectiveness of the stent graft, and increases the difficulty of matching the elongation behavior of the fabric graft, to that of the stent.
More particularly, the twisted multifilaments are tightly packed, yielding packing factors (or packing ratios) in the range of 0.7-0.9. Because of the tightly packed yarns, the tubular fabric graft has a tendency to kink when bent. The tightly packed filaments leave an insufficient void throughout the yarn cross-section for tissue ingrowth, reducing the effectiveness of long-term fixation. Further, the tightly packed yarn cross-section does not adjust itself in shape to accommodate axial elongation, thus limiting the radial contraction/axial elongation capability of the graft. The circular yarn cross-section further limits the elongation capability, because of its particular resistance to adjustments in shape.
Other disadvantages arise from the circular yarn cross-section. The yarn diameter determines the minimum thickness of the graft fabric. Yarn coverage typically is below 80 percent without additional compacting, and a fabric porosity usually is above 70 percent, again without additional compacting.
Several prostheses constructions had been suggested for composite braided structures that combine different types of strands, e.g. multifilament yarns, monofilaments, fusible materials and collagens. Examples are found in International Patent Publications No. WO 91/10766, No. WO 92/16166, No. WO 94106372, and No. WO 94/06373. Further, a highly favorable combination of strength, resilience, range of treatable lumen diameters and low permeability has been achieved by two-dimensionally woven and three-dimensionally woven composite devices featuring textile strands interbraided with either selectively cold-worked or independently thermally set structural strands, as disclosed in U.S. patent applications Ser. Nos. 08/640,062 and 08/640,091, both filed Apr. 30, 1996 and assigned to the assignee of this application. Although such devices are well suited for a wide range of procedures, there are costs and complexities inherent in interweaving different types of strands. Certain desirable modifications, e.g. providing selected areas of the device with only structural strands, are difficult.
All references cited herein, including the foregoing, are incorporated herein in their entireties for all purposes.
Therefore, it is an object of the present invention to provide a prosthesis structure that affords the advantages of stents and grafts, yet does not require an interbraiding of the structural strands characteristic of stents and the textile strands characteristic of grafts.
Another object is to provide a process for manufacturing a combination stent graft in which a structural layer and a low-permeability fabric layer undergo radial and axial enlargements and reductions, yet remain integrally bonded to one another.
It is a further object of the invention to provide a stent graft construction that facilitates selective alternate axial positioning of open-mesh areas and covered areas for shunting blood flow, to customize stent grafts for particular uses.
Yet another object is to provide, in a prosthesis featuring two or more layers formed of braided strands and of different materials, an effective bond to ensure that the layers remain integrally connected through radial expansions and contractions.
Another object is to provide a fabric graft that exhibits low permeability due to a high yarn coverage and low fabric porosity, in combination with a yarn cross-sectional porosity sufficient to enable tissue ingrowth.
A further object is to provide a stent graft with a fabric graft that is thinner and more closely matches the elongation behavior of the stent, yet affords acceptably low permeability.
To achieve the above and other objects, there is provided a process for making a stent graft, including:
a. forming a plurality of structural strands into a tubular open-mesh latticework adjustable between a nominal state and a radially-reduced axially-elongated delivery state according to a first relationship of radial reduction versus axial elongation;
b. forming a plurality of compliant textile strands into a tubular sleeve adjustable between a nominal state and a radially-reduced axially-elongated delivery state according to a second relationship of radial reduction versus axial elongation substantially equivalent to said first relationship, said latticework and said sleeve having substantially the same size and shape when in their respective nominal states;
c. applying an adhesive to at least one of said latticework and sleeve over at least a portion of the axial length of said at least one of the latticework and sleeve;
d. disposing a selected one of the latticework and sleeve within and axially aligned with the other of said latticework and sleeve so that said other surrounds said selected one, then bringing the latticework and sleeve into an engagement; and
e. maintaining the latticework and sleeve in said engagement for a time sufficient for the adhesive to bond the latticework and sleeve into a composite stent graft.
The step of applying an adhesive preferably involves a curable adhesive, applied uncured to the chosen one of the latticework and sleeve. Then, maintaining the latticework and sleeve in their engagement may further involve curing the adhesive to form the bond. A preferred manner of disposing one of the latticework and sleeve within the other is to adjust the selected one to reduce its radius below that in its nominal state, axially align it within the other while the other remains in its nominal state, then radially expand the selected one to achieve engagement of the latticework and sleeve.
Preferably the structural strands are interbraided in first and second sets of helices, running in opposite directions about a common longitudinal axis, and parallel to form a first braid angle with the latticework in its nominal state. The textile strands of the sleeve preferably are similarly braided in two sets of oppositely directed helices and at a second braid angle with the sleeve in its nominal state. The first and second braid angles preferably are within about 5 degrees of one another, and more preferably are within 3 degrees of each other. In a helical weave, the braid angle is an important factor in determining the degree of radial reduction for a given amount of axial elongation. Thus, the latticework and sleeve, having substantially similar braid angles and substantially the same size and shape in their nominal states, behave according to substantially the same relationship of radial reduction versus axial elongation. Accordingly, there is no tendency in the latticework to tear free of the sleeve due to its more rapid axial elongation for a given radial reduction. Conversely, there is no unwanted increase in bending stiffness due to an axial elongation of the sleeve that exceeds that of the latticework.
The latticework and sleeve are formed independently before their joinder. Structural strands forming the latticework and textile strands forming the sleeve are wound helically about respective mandrels at their respective braid angles, then thermally set, which in the case of metallic structural strands includes age-hardening. The latticework and sleeve are joined to one another with a curable adhesive, preferably a siloxane polymer. The polymer may be dissolved in a liquid organic solvent and applied to the latticework as a spray that leaves a silicone coating or residue when the solvent evaporates. The coated latticework is radially compressed and inserted axially within the sleeve, then expands to engage the sleeve. The bond is completed by heating the latticework and sleeve sufficiently to cure the adhesive. Alternative adhesives include the polycarbonate urethanes disclosed in U.S. Pat. No. 5,229,431 (Pinchuk).
There are several approaches to matching the respective braid angles. In one approach the latticework is formed on a mandrel smaller than the mandrel used for the sleeve, and the structural strands and textile strands are wound at the same braid angle. Alternatively the respective mandrels can have the same diameter. Then, the textile strands are wound at a braid angle slightly less than that of the structural strands. Then, as the sleeve undergoes the slight radial enlargement necessary for accommodating (surrounding) the latticework in the nominal state, its braid angle increases toward a closer match with the braid angle of the latticework.
Further in accordance with the present invention, there is provided a body insertable stent graft. The stent graft includes an expandable stent formed of a plurality of interconnected structural strands, adjustable between a nominal state and a radially-reduced axially-elongated delivery state. The stent graft also includes a tubular sleeve formed of a plurality of interwoven textile strands, adjustable between a nominal state and a radially-reduced and axially-elongated delivery state. Each of the textile strands is a multifilament yarn in which the multiple filaments have a surface twist angle of at most about 15 degrees. The sleeve and the latticework have substantially the same radii when in their respective nominal states. An attachment component fixes the latticework and the sleeve together, in a selected axial alignment with one another, engaged with one another and with a selected one of the latticework and sleeve surrounding the other, whereby the latticework structurally supports the sleeve.
According to another aspect of the invention, the yarns are formed to define a non-circular cross-section with major and minor axes, with an aspect ratio (major axis:minor axis) of at least about 2.
The flatter yarns with substantially untwisted filaments provide a fabric sleeve improved in the following respects: elongation behavior that more closely matches the elongation behavior of the stent; thinner walls for a reduced delivery profile; smaller interstices between yarns achieve lower permeability; and higher yarn cross-section porosity to allow tissue ingrowth.
Although the sleeve surrounds the latticework in the more preferred approach, an alternative construction features a sleeve surrounded by the latticework. In this latter case, the sleeve should be formed on a mandrel as large as the mandrel used to form the latticework, promoting a better bond with a sleeve that tends to elastically radially expand against the surrounding latticework. According to another alterative, two polymeric sleeves are employed, with the latticework sandwiched between an exterior sleeve and an interior sleeve. A variety of enhancements are provided within the scope of the present invention, e.g. incorporating one or more radiopaque strands in the latticework or sleeve, incorporating bioabsorbable materials, providing axial runners to enhance resistance to radial compression, and coating the completed stent graft or individual strands, to reduce deployment forces and lower the inflammatory response of tissue to the implanted device.
Thus in accordance with the present invention, a stent graft incorporates a structural latticework and low-permeability sleeve, independently formed and integrally connected to simultaneously provide the structural advantages of a stent and the low permeability of a graft. The latticework and sleeve are matched to undergo substantially the same degree of radial contraction for a given axial elongation. This matching, combined with an adhesive bond of the sleeve to the latticework, ensures that the stent graft radially expands and contracts as a unitary body, despite being composed of different structural and textile layers and despite the absence of an interweaving between the different layers or between the different types of strands.